Actually, a car service is going to run about the same as a cab...and a car service is much nicer. They will meet you at baggage claim, help with your bags, and you get to ride in a nice, clean car vs looking for a cab, after lugging your bags out to the curb.
Then, when it's time to return to the airport, you know that car service will be there at the appointed time.
A car service will run you about $110-120, not including tips, and a cab will run about $60 each way, again not including tip.